Surgical stents have long been known which can be surgically implanted into a body lumen, such as an artery, to reinforce, support, repair or otherwise enhance the performance of the lumen. For instance, in cardiovascular surgery it is often desirable to place a stent in the coronary artery at a location where the artery is damaged or is susceptible to collapse. The stent, once in place, reinforces that portion of the artery allowing normal blood flow to occur through the artery. One form of stent which is particularly desirable for implantation in arteries and other body lumens is a cylindrical stent which can be radially expanded from a first smaller diameter to a second larger diameter. Such radially expandable stents can be inserted into the artery by being located on a catheter and fed internally through the arterial pathways of the patient until the unexpanded stent is located where desired. The catheter is fitted with a balloon or other expansion mechanism which exerts a radial pressure outward on the stent causing the stent to expand radially to a larger diameter. Such expandable stents exhibit sufficient rigidity after being expanded that they will remain expanded after the catheter has been removed.
Radially expandable stents come in a variety of different configurations to provide optimal performance to various different particular circumstances. For instance, the patents to Lau (U.S. Pat. Nos. 5,514,154, 5,421,955, and 5,242,399), Baracci (U.S. Pat. No. 5,531,741), Frantzen (U.S. Pat. Nos. 5,718,713, 5,741,327, 5,746,691), Gaterud (U.S. Pat. No. 5,522,882), Gianturco (U.S. Pat. Nos. 5,507,771 and 5,314,444), Termin (U.S. Pat. No. 5,496,277), Lane (U.S. Pat. No. 5,494,029), Maeda (U.S. Pat. No. 5,507,767), Marin (Patent No. 5,443,477), Khosravi (U.S. Pat. No. 5,441,515), Jessen (U.S. Pat. No. 5,425,739), Hickle (U.S. Pat. No. 5,139,480), Schatz (U.S. Pat. No. 5,195,984), Fordenbacher (U.S. Pat. No. 5,549,662), and Wiktor (U.S. Pat. No. 5,133,732), each include some form of radially expandable stent for implantation into a body lumen. Other prior art stents are compiled in the Handbook of Coronary Stents, Second Edition, produced by the Rotterdam Thoraxcenter Interventional Cardiology Group.
Most of these prior art stents suffer from undesirable axial contraction when radially expanded. Stents can be made to resist axial contraction upon radial expansion by including axial elements therein extending continuously from a first end of the stent to a second end of the stent. However, such continuous axial elements tend to make the stent stiff and exhibit less flexibility characteristics than needed to allow the stent to be easily passed through tortuous arterial pathways or other tightly curving body lumens effectively. Some of these prior art stents, such as the stents described in the patents to Frantzen resist axial contraction upon radial expansion by locating axial elements offset from each other and within troughs of adjacent circumferential elements. While flexibility does improve somewhat by offsetting such axial elements, additional flexibility is often needed.
It is known to provide a combination of trough-to-trough axial elements alternating with curved axial elements oriented in a crest-to-crest fashion, such as with the stents described in the patents to Frantzen. The trough-to-trough axial elements resist axial contraction upon radial expansion of the stent and the crest-to-crest axial elements are curved to allow for some flexibility in the stent. Because the flexible axial elements extend crest-to-crest, these elements do not resist axial contraction of the stent upon radial expansion, but rather rely on the trough-to-trough axial elements. Accordingly, a need exists for a surgical stent which includes axial elements which both extend in a trough-to-trough fashion and also include flexibility characteristics in a single axial element.
Prior art stents have additionally suffered from poor visibility when viewed with a medical imaging device, such as a fluoroscope. During surgery, a surgeon will typically view the stent positioning procedure with a fluoroscope or other imaging device. Stents made from stainless steel, while adequate in most respects, are particularly difficult to view because they do not appear with a high degree of contrast relative to adjacent body tissues when viewed with a fluoroscope or other medical imaging device. This poor radiopacity is due partially to the particular radiopacity characteristics of stainless steel and also to the geometric configuration of the stent. Typical prior art stents are made from thin elements or wires with a significant amount of open space there between. Hence, a relatively small amount of stent material is present for a given area and this low density of stent material tends to decrease the radiopacity of the stent.
Known prior art methods for enhancing the radiopacity of a surgical stent include plating at least portions of the stent with chemical elements having a higher radiopacity or adding additional structural elements to the stent which are formed of a radiopaque chemical element so that the structural elements will be more visible. These prior art radiopacity enhancing techniques are inadequate due to the complexity involved in attaching or plating radiopaque material to the stent. Additionally, unless a proper amount of radiopaque material is added to the stent, the stent can return too strong of an image on the fluoroscope or other medical imaging device, causing the stent to obscure adjacent bodily tissues and decrease the surgeon's ability to properly locate the stent. Accordingly, a need exists for a simple way to add a radiopaque marker to a stent which enhances the radiopacity of portions of the stent just enough for clear viewing with a medical imaging device.